Lens system for optical reader

Abstract

First to third lenses are arranged in order to have a common optical axis. The first and second lenses provide a combined lens structure for shifting an object point along the common optical axis by moving one of the first and second lenses along the common optical axis, while an image point is formed in accordance with the shifted object point by the third lens. A spot diameter is kept constant at the image point by shifting the object point.

What is claimed is: 
     
       1. A lens system for an optical reader, comprising: first, second and third lenses having respective positive focal lengths and arranged in order to have a common optical axis, said first lens being movable along said common optical axis, and said second and third lenses providing a reverse telescope structure, whereby an image point formed by said third lens is movable along said common optical axis, and a substantially constant spot diameter is realized at said image point;   wherein said first to third lenses meet equations defined below,   f.sub.1 =(πW.sub.0 W.sub.3 /λ) [z/(b.sub.2 -b.sub.1)].sup.1/2 ; and       f.sub.2 =f.sub.3 [z/(b.sub.2 -b.sub.1)].sup.1/2       where f 1  is a focal length of said first lens, f 2  is focal length of said second lens, f 3  is a focal length of said third lens, W 0  is a diameter of a beam incident to said first lens, W 3  is a diameter of a converged beam at said image point, λ is a wave length of said beam, z is a displacement distance of said first lens, b 1  is a distance between said third lens and said image point formed at the nearest position from said third lens, and b 2  is a distance between said third lens and said image point formed at the farthest position from said third lens.   
     
     
       2. A lens system for an optical reader, comprising: first and second lenses having respective negative focal lengths and a third lens having a positive focal length, arranged in order to have a common optical axis, said second lens being movable along said common optical axis, and said first lens and said third lens providing a reverse telescope structure, whereby an image point formed by said third lens is movable along said common optical axis, and a substantially constant spot diameter is realized at said image point;   wherein said first and second lenses and said third lens meet equations defined below,   f.sub.1 =(πW.sub.0 W.sub.3 /λ) [z/(b.sub.2 -b.sub.1)].sup.1/2 ; and       f.sub.2 =f.sub.3 [z/(b.sub.2 -b.sub.1)].sup.1/2       where f 1  is a focal length of said second lens, f 2  is a focal length of said first lens, f 3  is a focal length of said third lens, W 0  is a diameter of a beam incident to said first lens, W 3  is a diameter of a converged beam at said image point, λ is a wave length of said beam, z is a displacement distance of said second lens, b 1  is a distance between said third lens and said image point formed at the nearest position from said third lens, and b 2  is a distance between said third lens and said image point formed at the farthest position from said third lens.   
     
     
       3. A lens system for an optical reader, comprising: first, second and third lenses arranged in order to have a common optical axis, said first lens being movable along said common optical axis to shift an object point, and said third lens forming an image point in accordance with said object point, whereby said image point is movable along said common optical axis, and a substantially constant spot diameter is realized at said image point; wherein   said first to third lenses each have a positive focal length.   
     
     
       4. A lens system for an optical reader, comprising: first, second and third lenses arranged in order to have a common optical axis, said second lens being movable along said common optical axis to shift an object point, and said third lens forming an image point in accordance with said object point, whereby said image point is movable along said common optical axis, and a substantially constant spot diameter is realized at said image point; wherein   said first and second lenses have a negative focal length and said third lens has a positive focal length.